This invention relates to testing the imbibition of fluid by a porous material which already contains another fluid. Although the invention is not necessarily limited to a specific application, it is useful for testing the imbibition of fluid by the rock of an oil reservoir. Such testing may be required when it is intended to recover oil by injecting another fluid.
Injecting a fluid to displace oil and drive it out of a reservoir may be done in the context of producing heavy oil from an underground reservoir or in the context of enhanced oil recovery after initial production has partially depleted the reservoir. It is desirable and indeed may be it essential that it is energetically favorable for the injected fluid to enter the rock pores.
For instance, oil reservoirs which are naturally fractured carbonate rock often comprise high-permeability fractures in low-permeability matrix rock. A water-flooding process may be used to drive oil from the reservoir. In such a process, water is pumped into the reservoir to displace the oil, forcing it away from the injection well towards adjacent wells from which it is produced. If the matrix is water-wet (i.e., it is energetically favorable for water to enter the matrix) then water-flooding can be an effective means of recovering more oil. However, if the matrix is oil-wet (i.e., it is energetically unfavorable for water to enter the matrix) water-flooding is not effective because the water flows in the fractures and cannot be forced through the porous rock matrix.
In such cases oil can be produced by pumping a fluid (the imbibant) which is spontaneously imbibed by the rock matrix (the imbibor). An aqueous imbibant fluid may be a surfactant solution but other fluids such as brines may also be considered. Spontaneous imbibition of an aqueous solution into an oil-bearing rock matrix depends upon parameters such as the wettability of the rock, the interactions between the fluid and the rock, and the interactions between the fluid and the in-situ oil. These interactions can be difficult to quantify.
When it is intended to use an injected fluid to displace oil, it is desirable to test imbibition and optimize the fluid for the reservoir rock and the existing reservoir fluids. If the imbibant is a surfactant solution and the imbibor is an oil-bearing reservoir rock (which also is likely to contain a formation brine), the surfactant may be absorbed into the oil or the formation brine, onto the oil-imbibant interface, or onto the imbibor pore surface.
Tests to look at isolated interactions (e.g., measurements of surface tension, or measurements of surfactant adsorption onto a mineral surface) may be carried out and the results used to predict the combined effect of the interactions. However, it is also useful to make a direct experimental test of imbibition of fluid by the porous reservoir matrix. This is customarily done by the Amott cell test which is carried out by immersing a sample of the porous medium in the fluid and measuring the amount of fluid that it imbibes or the amount of oil that is produced from the sample. However, a rock core (or slice of a rock) is needed for each test which may last several days (not including preparation time). These tests can be time consuming and may need a lot of material.
There have been proposals for experimental methods in which a droplet of water is placed on a porous material which is dry, so that its pore space is filled only with air, and the time for the droplet to penetrate into the porous material is measured. This has been proposed as a measurement of the wettability of the porous material which is just one of the parameters mentioned above which affect imbibition.
The present inventor points out that the result in such a test is affected by the area of contact between water droplet and the porous material and cannot provide a true measurement of imbibition.